Many processes exist wherein a hot gaseous medium is produced which contains particulate material that must be separated from the gaseous medium, either to prevent pollution, or to remove hazardous material. High temperature filtration of particulates has become an important component in many emerging technologies. Advanced coal conversion technologies, such as fluid bed gasification and combustion, are dependent upon the successful removal of particulates at temperatures in the range of about 500.degree. to 1100.degree. C. Other applications which benefit from high temperature filtration range from gas cleaning for biomass gasification to power generation from the incineration of municipal solid wastes. These applications require the removal of particulates from gas streams at high temperature so that process equipment, such as rotating machinery and heat exchange surfaces, remain functional and efficient throughout the use of such equipment.
Rigid ceramic filters are currently being developed and used for separating entrained particles, such as flyash or char, from the hot gases produced in these energy generating systems and industrial processes. In these hot gas filtration systems, the ceramic filter serves as the only filter device for trapping undesirable particles contained in the flow of hot gases which pass through the filtration system.
One type of ceramic filter element, the cross flow filter, is described in U.S. Pat. No. 4,343,631--Ciliberti, which is incorporated herein in its entirety by reference, and is assigned to the assignee of the present invention. The cross flow filter comprises several layers of porous ceramic membranes joined together in such a manner as to maximize filter area per unit volume. Particle laden gases pass into dirty side channels of the filter and then through the filter membranes where the particles are deposited as cake on the surface of the membranes within the dirty side channels. The cleaned gases then pass into and through the clean side channels of the filter and subsequently exit the filter. Generally within the filter system, a plurality of such filter elements are connected to a single plenum pipe through which the filtered clean gas passes after flowing through the filter elements.
The filter elements are periodically cleaned by providing a pulse of high pressure gas which is pumped through a pulse cleaning pipe in flow communication with the plenum pipe. The pulse of high pressure gas causes reverse flow through the filter elements which dislodges the cake of particles trapped by the filter elements such that the cake falls out of the filter elements and is collected and disposed of through a discharge point in a known manner. The high pressure gas used to clean the filters is usually cold due to the known difficulties of pulsing high temperature gas with existing valves.
A second type of rigid ceramic filter is referred to as a candle filter and comprises a hollow cylinder which is closed at one end and flanged at the other for attachment to a tubesheet or blowback plenum, into which cleaned gas passes during the filtration cycle. Particle laden gas passes through filter elements around the cylinder such that the particles are trapped in the filter and the clean gas flows into the hollow center of the candle and out through the open end. In the system, a plurality of these candles are connected to a plenum pipe such that the clean gas from the plurality of candles flows into the plenum pipe. A pulse of cold gas is periodically blown into the candles for dislodging the cake from the filter elements.
Candle-type ceramic barrier filters of the general type discussed above are disclosed in U.S. Pat. No. 4,973,458--Newby et al.; U.S. Pat. No. 4,812,149--Griffin et al.; U.S. Pat. No. 4,764,190--Israelson et al.; U.S. Pat. No. 4,735,635--Israelson et al.; and U.S. Pat. No. 4,539,025--Ciliberti et al., each of which is incorporated herein in its entirety by reference.
However, problems have been recognized in the use of hot gas filtration devices comprising rigid ceramic filter systems. Since these types of filter devices comprise porous ceramic materials which are subjected to high temperature corrosive environments, with fluctuations in temperature, one or more of the individual filter elements in the system can break under the influence of these conditions. Moreover, since the pulse of high pressure gas is cold, the elements are subjected to severe changes in temperature such that the filter elements are further prone to breaking. Where one or more of the filter elements in the system breaks, leaving a hole in the filter, an open path through the filter vessel is available such that the flow of gas through the hole is limited only by the relatively small flow resistance of the orifice left by the missing pieces of ceramic material. Thus, dirty particles remain in the gas after passing through the filtration system, resulting in a substantial decrease in the overall efficiency of the system and adverse effects on the environment.
Although research and development efforts are currently being carried out to improve the ceramic materials of these filtration systems so as to reduce the susceptibility of the components to breakage under such adverse conditions, there is presently a need for a fail-safe backup filter device for collecting the particles of dirty gas which pass through the filter vessel in the event of a break in one or more of the individual filter elements. Also, since a fail-safe backup filter in accordance with the present invention provides sufficient thermal capacity and heat transfer area for minimizing the adverse effects of severe temperature fluctuations on the filter elements during periodic reverse flow pulsations, the present invention provides a currently available solution to the problems associated with pulse flow cleaning of known filtration systems.